The present invention concerns the plating and finishing of metal pieces immersed in a bath and in particular concerns an improved system for galvanic coating or finishing treatments on metal pieces, and in particular small metal items.
In conventional systems for galvanic treatment, for example for zinc plating or galvanizing metal pieces, the pieces are loaded into tumbling barrels, or xe2x80x9ctumblersxe2x80x9d, having a cylindrical or prismatic shape and set with the axis horizontal, the said barrels having perforated walls and an openable door. Each barrel is carried on a supporting frame and is free to turn about its own axis.
The system comprises a set of baths arranged in a line, in which the frame-and-barrel assemblies loaded with the pieces to be treated are immersed. In each of the said baths, a particular stage of the treatment or a stage preliminary to the treatment is carried out, such as washing, degreasing, pickling, de-activation, and zinc coating (galvanization). Generally, a number of barrels are treated simultaneously in the same bath. The barrels are set side by side in each bath in a stationary position, and each barrel is moved from one bath to the subsequent one by means of an overhead travelling crane, which lifts it up, displaces it according to a direction referred to as direction of advance, and lowers it into the new position. The direction of advance and the direction defined by the axes of the barrels are usually orthogonal with respect to one another.
Unavoidably, the barrel being lifted up and undergoing displacement drips onto the underlying barrels in the treatment baths, onto the suction hoods, and onto the anodic bars of the electrodeposition baths, thus causing serious contamination of the solutions. It is obvious that, if an acid drips into a washing bath or, on the other hand, if washing liquid drips into a galvanizing bath, the corresponding washing and galvanizing processes will be of a lower quality.
Another serious drawback with traditional systems, which considerably limits productivity in terms of number of tumblers treated per hour, is due to the fact that, in order to lift out a barrel at the end of the process, an overhead travelling crane must traverse a path in the two directions of movement, i.e., loading and unloading, and the time required ranges between the minimum time necessary for covering a spatial unit occupied by a single barrel and the maximum time necessary for covering n units occupied by n tumblers. This results in a discontinuity in the times of deposition of the metal which has the unfavourable effect of an unevenness of the thicknesses deposited.
Another drawback linked to known systems is the way in which the anodes are distributed. In fact, the anodic bars are set transversely with respect to the baths and parallel to the axes of the barrels, between adjacent barrels, and this reduces the quality of the treatment. In particular, the fact that an anodic bar is shared between two adjacent tumblers produces the undesirable effect of conveying the greater number of cations, and hence more metal, towards the tumbler that presents higher conductivity, at the expense of the other tumbler. In addition, replacement of the anodic bars is particularly inconvenient and dangerous, since the operator in charge must climb onto the baths to carry out periodic replacement.
Another negative effect, in traditional systems, is the difficulty of eliminating emissions of fumes and vapours on account of the long paths traversed, outside the baths, by the barrels soaked with the solutions used in the various processes. In addition, a possible means of aspiration of fumes in traditional systems can be placed only in the limited space that is transverse to the direction of advance between adjacent barrels, and consequently must be equipped with powerful suction appliances, with high costs and only modest results.
Furthermore, in traditional systems control of the current is carried out by placing a current rectifier for each position of electrolytic plating, with an enormous economic expenditure owing to the number of connections and the number of current generators required for electrolytic processes.
The purpose of the present invention is to provide a system for the treatment of pieces that is able to overcome the drawbacks referred to above, and in particular, to prevent dripping of the barrels during their transportation from one bath to another, or rather to prevent the dripping of substances into baths in which the presence of these substances is undesirable.
The above purpose and other purposes are brilliantly achieved by means of a system having the characteristics of the independent claim 1, and by means of a method according to claim 17. Further characteristics of the invention are specified in the dependent claims.
The invention achieves the purposes set, i.e., prevents dripping of the barrels during their progress through the cycle, reduces the times of transportation of the barrels during their progress, prevents discontinuity and unevenness of treatment, facilitates periodic replacement of the anodic bars, enables installation of suction hoods over a wide area of the treatment baths (in that it is not necessary for the overhead travelling crane to have access to the entire extent of the treatment baths), and simplifies the connections for the current required for the electrolytic processes.